Question
I want to show different ads (my own ads, not any ad network's) according to the demographics of my website visitors (different age groups, different interests, purchase behavior, gender etc). I know there are companies which provide audience data, I'm looking for an easier plug and play and affordable solution.
Answer
Continuing on what Michael said...
I'm not aware of a ready solution that will do all that you want.. Though it is the line of development we have for our own WordPress plugin - Icegram - http://www.icegram.com.
Icegram shows different ads / call to actions / lead capture forms to users. And includes flexible targeting rules. You can actually create your own plugin that adds more targeting rules. So you can extend Icegram to support targeting by demographics you mentioned.
Icegram is free. With paid addons. You may want to give it a try.